Known methods of detecting knock include measurement of a pressure in, or sound from, the engine cylinders by appropriate detectors. However, it is difficult to distinguish signals produced by knock from other sounds which occur normally in the engine, and those methods are therefore usually unsatisfactory.
During detonation of a fuel/air mixture in a cylinder of an engine, ions are produced and an ionic current can be detected by measuring the current across a probe gap (e.g. conveniently provided by the gap between the electrodes of a spark plug in the cylinder) as described in the article "Studies of Engine Combustion Processes by Ionization Current" by K. Iinuma in Bull. of JSME, Vol. 4, No. 14, pages 352 to 357, 1961.
When knock is produced by a combustion event in a cylinder of an engine, high-frequency oscillations are generated in the ionic current. These oscillations, which can also be generated by events other than knock, can be filtered and then interpreted to determine whether they contain features which are characteristic of knock.
For example, rotation of the engine distributor can lead to spark-overs in the distributor (due to residual energy in the ignition coil) even after decay of the ignition voltage. The spark-overs can cause diodes in the measuring circuit to switch over from a blocking state to a conducting state, producing a steep signal rise. The steep signal rise includes many frequencies, including those which are the same as those associated with knock. Interference thus occurs, and it is often impossible to determine whether certain characteristics of the filtered signal result from knock.